1. Field of Invention
This invention is directed to catalysts useful in the conversion of carbon oxides. Specifically it is directed to catalyst useful in the conversion of carbon oxides and water for the production of hydrogen.
2. Prior Art
Synthesis gas represents one of the most important feedstocks of the chemical industry. It is used to synthesize basic chemicals, such as methanol or oxyaldehydes, as well as for the production of ammonia and pure hydrogen. However, synthesis gas produced by steam reforming of hydrocarbons does not meet the requirements for further use in some cases because the CO/H.sub.2 ratio is too high. It is therefore industrial practice to reduce the CO content by conversion with steam.
Hydrogen is an indispensable component for many petroleum and chemical processes. Refineries in the petroleum industry, and methanol and ammonia plants in the chemical industry consume considerable quantities of hydrogen during processes for the production of gasoline and fertilizers. As environmental regulations demand cleaner, renewable and non-polluting processes and products, most of the hydrogen balances at petroleum refineries are becoming negative. As laws mandate lower aromatics in gasoline and diesel fuels, H.sub.2 is now consumed in aromatic saturation and thus, less H.sub.2 is available as a by-product. At the same time, H.sub.2 consumption is increasing in hydro-treating units in the refineries because many of these same laws require low sulfur level in fuels.
Hydrogen is primarily obtained by steam reforming methane or mixture of hydrocarbons, a reaction which produces hydrogen, carbon dioxide and carbon monoxide. To improve H.sub.2 yield and also the operating efficiency of carbon monoxide conversion, the water gas shift reaction is extensively used in commercial hydrogen or ammonia plants. The reaction can be described as:
CO+H.sub.2 O CO.sub.2 +H.sub.2 H=-9.84 Kcal/mol at 298.degree. K. PA1 (1) Modify the catalyst into a suitable morphological form with typical particle size from 20-200 microns; PA1 (2) Modify the catalyst with desirable pore structure with peak of the pore distribution at around 50-200 angstroms and the total pore volume at 0.2-0.4 cc/g; PA1 (3) Modify the catalyst with desirable BET surface area in the neighborhood of 40 to 200 m.sup.2 /g; PA1 (4) Construct and reinforce the framework of the catalyst so that the catalyst can maintain its physical integrity and strength under certain mechanical, thermal, and/or reaction forces encountered in industrial applications; and PA1 (5) Separate the Cu/ZnO crystallites and put appropriate space among them so that the Cu/ZnO pair is able to be well dispersed throughout the catalyst structure. PA1 (6) Allow electron transfer between copper and the promoter and enhance interaction among the components.
For maximum H.sub.2 yield and CO conversion efficiency, the water gas shift reaction is usually carried out in two stages: at high temperatures typically 350-400.degree. C. and at low temperature typically 180-240.degree. C.
While lower temperatures favor more complete carbon monoxide conversion, higher temperatures allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature reaction unit for bulk carbon monoxide conversion and heat recovery, and a low temperature reaction unit for final carbon monoxide conversion.
Chromium-promoted iron catalysts are normally used in the first stage at temperatures above about 350.degree. C. to reduce the CO content to about 3-4% (see, for example, D. S. Newsom, Catal. Rev., 21, p. 275 (1980)). As is known from the literature (see for example, H. Topsoe and M. Boudart, J. Catal., 31, p. 346 (1973)), the chromium oxide promoter combines two functions. In the first place, it serves to enhance catalytic activity and in the second place, it acts as a heat stabilizer, i.e., it increases the heat stability of magnetite, the active form of the catalyst, and prevents unduly rapid deactivation under conditions of technical use.
Unfortunately, when chromium is used, especially in hexavalent form, expenditures must be incurred to guarantee worker safety both during production and later handling of the catalyst, and health hazards cannot be fully ruled out despite considerable effort. In addition, the spent catalyst ultimately poses a hazard to man and the environment and must be disposed of with allowance for the provisions in force for toxic waste.
The commonly used catalysts for water gas shift reaction at low temperature (or so-called low temperature shift reaction) in industry contain copper oxide, zinc oxide and aluminum oxide. Because these catalysts operate at relatively low temperature, they generate equilibrium carbon monoxide concentrations of less than 0.3% in the exit gas stream over an active low temperature shift catalyst. However, performance of carbon monoxide conversion and hydrogen yield gradually decreases during normal operations as a result of deactivation of the catalyst. This deactivation is caused by poisoning, generally from traces of chloride and sulfur compounds in the feed and the hydrothermal environment of the reaction. The rate of the hydrothermal deactivation, in particular, is dependent on reaction conditions such as temperature, steam to gas ratio and composition of the feed gas mixture, and is closely dependent on the formulation and manufacturing process of the catalyst.
A typical composition of a low temperature shift catalyst is comprised of from 30 to 60% of CuO, 20 to 50% of ZnO and 5-40% of Al.sub.2 O.sub.3. The catalyst is usually made by either co-precipitation of metal salts (nitrate, sulfate, or acetate), thermal decomposition of metal complexes, or impregnation of metal salt onto a carrier. Depending on the preparation conditions (pH, temperature, addition rate and composition), one or several of the following mixed copper/zinc hydroxy carbonate phases are present in the precursor of the catalyst: (a) malachite Cu.sub.2 CO.sub.3 (OH).sub.2, (b) rosasite (Cu,Zn).sub.2 CO.sub.3 (OH).sub.2, (c) hydrozincite Zn.sub.5 (CO.sub.3).sub.2 (OH).sub.6, (d) aurichalcite (Cu,Zn).sub.5 (CO.sub.3).sub.2 (OH).sub.6, and (e) hydrotalcite (Cu,Zn).sub.6 Al.sub.2 (OH).sub.16 CO.sub.3. The catalyst is then washed to remove foreign ions, dried and calcined at an appropriate temperature to form oxides. With appropriate precursors and preparation conditions, a mixed copper/zinc oxide phase rather than segregated cupric oxide and zinc oxide will be formed during calcination at 250-450.degree. C. The catalyst must be reduced with hydrogen at 100-300.degree. C. before being put on stream. During reduction, copper oxide in cupric form is reduced to either metallic copper or/and cuprous oxide.
It is well accepted that reduced copper is an active species for low temperature shift catalyst. The reaction is initiated by adsorption of water and carbon monoxide molecules, proceeds with dissociation of water, and completes with association of the adsorbed intermediates to form hydrogen and carbon dioxide. All the steps mentioned above are carried out on the surface of copper active sites. In general, copper-based catalysts are very susceptible to thermal sintering which results in a loss of copper surface area and therefore activity. This situation arises because of high dispersion of the reduced copper on the catalyst and high mobility of the highly dispersed copper crystallites. With the presence of steam under the reaction environment, a rapid loss of copper surface area occurs as a result of sintering. Under some extreme catalyst testing conditions (high steam to carbon ratio), 30 to 50% of the original copper surface area may be lost in a test of 10 to 15 days depending upon the formulation and preparation method. It is believed that an industrial low temperature shift catalyst copper is partially stabilized by zinc oxide and aluminum oxide. In the presence of significant partial pressures of steam as in the low temperature shift conditions, zinc oxide selectively adsorbs water. Migration and/or inclusion of copper into a zinc oxide matrix inhibits copper crystallite growth. In addition, zinc oxide protects copper from poisoning of chloride and sulfur. The appropriate incorporation of aluminum in the matrix of copper oxide and zinc oxide can further increase the hydrothermal stability of copper.
As mentioned above, standard catalysts for this conversion stage are based on Cu--Zn oxide. See, for example, U.S. Pat. No. 1,809,978. This type of catalyst has a major drawback of extremely low heat stability so that its use is essentially limited to temperatures below about 250.degree. C. Further catalyst developments have focused on conversion activity at lower temperatures. See U.S. Pat. No. 3,303,001.
U.S. Pat. No. 4,308,176 describes catalysts for conversion of carbon monoxide based on copper oxide and/or zinc oxide on aluminum oxide spinels, wherein the catalyst is improved by the incorporation of zinc oxide into the pores of the spinel structure. U.S. Pat. No. 3,922,337 discloses a low-temperature carbon monoxide shift catalyst containing copper and zinc oxides, wherein a sodium alkalized alumina improves resistance against halogen poisoning. See also U.S. Pat. No. 3,518,208.
European Patent No. 0 296 734 B1 discloses a copper containing catalyst for carbon monoxide conversion. The catalyst is formed from copper oxide and one or more other oxidic materials generally including zinc oxide. The catalyst may also contain oxides of at least one other element selected from the group of aluminum, vanadium, chromium, titanium, zirconium, thorium, uranium, molybdenum, tungsten, manganese, boron, and the rare earth elements. Preferably, alumina is used. The catalyst produced possesses high specific copper surface area outside the range of conventional, unpromoted copper/zinc catalyst. See also U.S. Pat. No. 4,711,773.
Copper-containing catalysts for low temperature shift conversion may also include a potassium component to suppress the formation of by-products, such as amines and methanol as disclosed in U.S. Pat. No. 5,128,307. A similar type of catalyst may also be "alkali doped" as disclosed in U.S. Pat. No. 5,021,233.
In addition, catalysts for the reaction of carbon monoxide with steam containing copper, zinc, and at least one metal selected from manganese and the metals of Groups II to V on the Periodic Table wherein those metals preferably include aluminum or magnesium, although titanium or zirconium or thorium can be used are disclosed in G.B. 1,131,631.
Catalysts for use in carbon monoxide shift reactions comprised of a copper/zinc/alumina precursor and another metal selected from the group of lanthanum, cerium, or zirconium are disclosed in U.S. Pat. No. 4,835,132.
U.S. Pat. No. 4,683,218 discloses a catalyst for a water gas shift reaction comprised of zinc, copper, an element from the lanthanum group and from the rare earth group.
Although copper is physically and physicochemically stabilized by both zinc oxide and aluminum oxide and attempts of further stabilization of the catalyst have been made as taught by prior art, sintering of copper crystallite is still a main cause for deactivation/aging of the catalyst, especially when there are very low concentration of poison in the feed. For example, the copper crystallite size of a fresh catalyst ranges from 30-100 angstroms in contrast with 100-1,000 angstroms over a discharged used catalyst from the plant. The known catalysts thus need to be improved with regard to activity and stability.
A major reason for lack of superior activity and hydrothermal stability over the known catalyst is due to lack of significant electronic modifications and interactions among copper, zinc oxide and aluminum oxide. One or several highly thermally stable component(s) is(are) thus needed to act as a promoter/stabilizer of the catalyst and it(they) should have one or several of the following functions:
The objective of the present invention thus is a catalyst for a CO conversion process that has superior activity and hydrothermal stability.
It is a further object of the present invention to produce a long life low temperature water gas shift catalyst.
It is a still further object of the present invention to prepare a catalyst for a CO conversion process that exhibits significant hydrogen production over the lifetime of the catalyst.
It is a still further object of this invention to produce a catalyst that can be used for other processes in which carbon oxides are converted to methane and/or methanol.